1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems of the traction type which are driven by a direct current motor.
2. Description of the Prior Art
Traction elevator systems suspend the elevator car on a plurality of wire ropes which pass over a traction sheave and are connected to a counterweight. The traction sheave is usually driven by an electrical drive motor, such as an A.C. induction motor via a reduction gear, or a D.C. motor, either directly or via a reduction gear, depending upon the contract speed of the elevator.
The mechanical system of the traction elevator, which consists of all rotational and translational inertia and cable spring compliance, behaves as a resonant system with very little damping. The oscillation frequency of the mechanical system ranges between about 3 and 15 hertz, and is a function of the aforesaid parameters, as well as the load in the elevator car, and the position and speed of the elevator car. Anything that perturbs the mechanical system at its resonant frequency can cause an annoying vertical oscillation of the elevator car, referred to as jitter.
The source of the perturbance which initiates the jitter may come from any one of a plurality of different sources. As discussed in my U.S. Pat. No. 3,749,204, jitter may be caused by electrical noise in the stabilizing signal applied to the velocity error signal in the control loop which determines the magnitude of the direct current voltage applied to the drive motor. U.S. Pat. No. 3,749,204 discloses an acceleration transducer for providing a stabilizing signal. A disadvantage of the acceleration transducer arrangement is that a variable amount of armature voltage feedback, (a parasitic signal in this case) is an integral part of the signal. The variability is due to the change in armature inductance with field strength and to armature resistance with temperature. The stabilizing signal provides some jitter suppression, but the required amount sometimes cannot be used because the system can become unstable at some higher frequency due to the parasitic signal. The present invention does not contain the parasitic signal and therefore allows a stronger signal to be applied to more effectively suppress the jitter with a smaller probability of causing instability at other frequencies. Also, the stabilizing function and the jitter suppression function are separated, which allows independent adustment to optimize stability and smoothness.
Unfortunately, the mechancal system may be shocked into resonance from other sources. For example, in those elevator systems in which the source of the adjustable direct current voltage is a static dual bridge converter, if the bridges are not switched precisely at zero current, a sudden torque change in the output shaft of the drive motor due to an abrupt armature current change may shock the mechanical system into resonance.
Another source of the perturbance, which may be produced with either a motor generator voltage source, or a static converter voltage source, is due to the relationship between the pole and other mechanical structure of the motor, the motor speed, and drive shear diameter. The output torque of the motor may be inherently perturbed due to its structure a predetermined number of times for each revolution of the motor, which for a predetermined motor speed and sheave diameter may translate to a perturbance frequency in the resonant frequency range of the mechanical system.
Thus, it would be desirable to be able to attenuate vertical oscillation of the elevator car, and prevent noticeable jitter, regardless of the perturbing source.